This application is an improvement over Grubbs U.S. Pat. No. 3,233,781 for Toner Control System. This patent shows a system wherein a pair of photoconductive devices are connected in parallel by resistances to form a bridge circuit having four resistance arms. The bridge is manually adjusted to null at the desired toner density by varying one of the resistance arms of the bridge. Photoconductive devices, however, are sensitive to temperature; and the equivalent circuit of such devices may be considered as a light responsive resistance shunted by a temperature responsive resistance. The null point of the Grubbs circuit will thus shift as a function of temperature unless the setting of the manually adjustable arm of the bridge happens to be such that equal amounts of light or luminous flux fall on the two photoconductive elements. However, where it is desired to vary the toner density to meet particular requirements, the null point of the bridge becomes increasingly temperature sensitive; and a manual adjustment which is satisfactory when a xerographic machine is first started may become unsatisfactory at a later time due to the increase in ambient temperature caused by operation of the machine.